1. Field of Application
The present invention relates to a DC-DC converter which applies feedback control of a switching element for setting an output voltage at a target value, and to a method of designing such a DC-DC converter.
2. Description of Related Art
Types of DC-DC converter are known, for converting an input DC voltage to an output DC voltage that is different from the input DC voltage, by driving a switching element by a PWM (pulse width modulation) signal to convert the input DC voltage to a voltage having a repetitively varying waveform, which is smoothed by a smoothing circuit to obtain the output DC voltage. In general, the smoothing circuit is a LPF (low-pass filter). The duty ratio of the PWM signal is controlled such as to maintain the level of the output DC voltage at a predetermined value. Such a type of DC-DC converter is generally referred to as a chopper-type of switching regulator.
With such a type of DC-DC converter, in order to reduce the amount of ripple in the output DC voltage and to reduce the danger of oscillation occurring in the feedback control loop (i.e., which is a negative-feedback loop), it is necessary to maintain a sufficiently large phase margin at the cut-off frequency of the smoothing circuit. The phase margin is the difference between the phase delay angle of the control loop and 180°, and the cut-off frequency is the frequency at which the loop gain of the control loop reaches 0 dB.
With regard to achieving a high degree of smoothing of ripple, it is preferable to use a smoothing circuit in which the capacitor has as small a value of ESR (equivalent series resistance) as possible, such as an organic high-molecular capacitor or a ceramic capacitor. This is due to the fact that when noise current components pass through the capacitor, the resultant voltage variations which occur across the ESR of the capacitor produce noise in the output DC voltage of the converter.
A generally used method of setting a high value of phase margin is to set the loop gain at high frequencies to become 0 dB before the phase delay angle reaches 180°, with this being referred to as the phase delay compensation method. Another method of setting a high value of phase margin is to advance the phase, i.e., a phase advance compensation method, for example as described in an Internet on-line document which was found by a search made on 15 Dec. 2006, URL:http://www.asahi-net.or.jp/˜bz9s-wtb/doc/power/N01/tip1c3b.pdf, “Know Control”, pages 6-8.
If a capacitor having a small value of ESR is used in the smoothing circuit of the control loop, then the effectiveness of ripple exclusion is enhanced, however as shown in FIG. 2, due to the fact that phase rotation becomes rapid, it is difficult to maintain a sufficient phase margin. Hence the problem arises that the stability of feedback control is lowered.
FIG. 2 is a Bode diagram showing DC-DC converter characteristics, with the full-line portion being a phase characteristic for the case in which a high-ESR capacitor is used in the smoothing circuit of the control loop of the DC voltage, and with the broken-line portion being a phase characteristic for the case in which a low-ESR capacitor is used.
If a phase compensation method is used, to achieve a sufficiently large phase margin, then this results in lowering of the loop gain, and may require the use of an additional special circuit for advancing the phase, so that the circuit configuration becomes complex.
If the phase delay compensation method is used, the problem arises that since high-frequency components are excluded from the feedback information (i.e., high frequency components of the feedback control signal are attenuated), there is a lowering of the speed of control response to variations in the input DC voltage of the converter. On the other hand, If the phase advance compensation method is used, the problem arises that it is extremely difficult to implement a circuit design whereby only the phase of the feedback control signal will be advanced, without affecting the loop gain.